1. Field of the Invention
The present invention relates to a motor control apparatus which supplies drive power to a motor by first converting input AC to DC for output and then converting the DC output back to AC for driving the motor.
2. Description of the Related Art
In a machine tool system, motors are provided one for each drive axis of the machine tool, and these motors are driven and controlled by a motor control apparatus. For example, a spindle motor is used for rotating the tool or work, and a servo motor is used for contouring control. The motor control apparatus commands and controls the motor speed, torque, or rotor position of each of the motors that drive the respective drive axes of the machine tool.
The motor control apparatus includes a converter which converts AC input to DC for output, an inverter which converts the DC output of the converter to produce an AC output of a desired frequency for driving the motor, and a numerical control part which outputs a motor operation command for controlling motor operation such as the speed, torque, or rotor position of the motor connected to the AC output side of the inverter.
As many inverters are provided as there are motors in order to drive and control the motors by separately supplying drive power to each of the motors connected to the respective drive axes of the machine tool. Each inverter incorporates an inverter control part which controls the AC output of the inverter so that the motor is operated in accordance with the motor operation command supplied from the numerical control part. On the other hand, usually only one converter is provided for the plurality of inverters in order to save on the cost and installation space of the motor control apparatus.
When supplying large AC power from the inverter to the motor, such as when driving the motor with a large output, large power also has to be supplied from the converter to the inverter, and as a result, therefore an excessive current may flow into the AC side of the converter. When, for example, a safety device is installed, if an output exceeding the allowable output of the converter is demanded by the inverter, the safety device is activated, issuing an alarm to stop the motor control apparatus in order to prevent a large current exceeding the allowable limit from flowing into the AC input side of the converter.
For example, for use in air-conditioning equipment, several types of devices that limit the AC current to be input to the air-conditioning equipment are proposed in order to prevent the activation of a power breaker provided on the AC input side of the equipment.
For example, Japanese Unexamined Patent Publication No. H04-106348 discloses a technique wherein, in air-conditioning equipment having a circuit that first converts an AC input to a DC output and then converts the DC output to AC for driving a compressor, a limit value is set on the input AC current and, when the input AC current exceeds the limit value, the limit value is lowered in a stepwise manner by an amount equal to the difference, thereby reducing the output frequency of the AC for driving the compressor, causing the compressor to run in a low performance state and thus limiting the AC current to be input to the air-conditioning equipment.
The above-cited Japanese Unexamined Patent Publication No. H04-106348, Japanese Examined Patent Publication No. H07-10198 and Japanese Unexamined Patent Publication No. 2010-233304, for example, each disclose a technique wherein, in air-conditioning equipment having a circuit that first converts an AC input to a DC output and then converts the DC output to AC for driving a compressor, the AC input current and DC output voltage of an AC/DC conversion circuit are monitored and, when the DC output voltage drops, the AC output frequency of a DC/AC converter is reduced, thereby causing the compressor to run at a low rotational speed and thus limiting the AC current to be input to the air-conditioning equipment.
In other fields than the above-described air-conditioning equipment field, there are also proposed devices for preventing the occurrence of an overcurrent at the power supply side due to excessive power drain by a motor connected to the AC output side. For example, in the field of electric vehicles, Japanese Patent No. 4111138 proposes a technique that limits the motor output to prevent failure when the delivery of power that exceeds the allowable load of the DC power supply is demanded from the motor.
If the motor control apparatus stops due to an alarm condition as earlier described, not only the entire operation of the machine tool equipped with the motors being driven under the control of the motor control apparatus, but the product being worked on is also significantly affected. For example, before restarting the motor control apparatus once stopped due to an alarm condition, first it is necessary to remove the event that caused the alarm condition, and then to check whether any problem has been caused to other machine tools due to the stoppage or whether any damage or defect has been caused to the product being worked on, and it is also necessary to move the stopped tool and change to their initial positions; restart the apparatus which takes considerable time, and human labor.
In order to prevent the stoppage of the motor control apparatus in an alarm condition, it has been common to design the motor control apparatus by employing a converter capable of delivering the power that is deemed necessary when all the inverters connected to the converter are operated at their maximum output power. When the converter selected in this way is employed, the converter can always supply sufficient output power, which serves to avoid a situation where an excessive current that exceeds the allowable limit flows into the AC input side of the converter. However, in the case of a machine tool whose operation seldom requires all the inverters to be operated simultaneously at their maximum output power, the capacity of the converter becomes larger than necessary, and hence the problem that the prior practice is disadvantageous in term of installation space and cost.
In one possible approach to solving this problem, when it is expected that all the inverters are not required to be operated simultaneously at their maximum output power, the maximum output required of each inverter is predicted from the expected motor operating conditions, and the converter is chosen to be able to provide enough power so that each inverter can output power that falls within the predicted maximum output. However, if the prediction is wrong, or if there arises a situation where the motor is operated outside the predicted range, the maximum allowable output of the thus chosen converter may be exceeded, in which case it is not possible to eliminate the possibility that an excessive current that exceeds the allowable limit flows into the AC input side of the converter, causing the motor control apparatus to stop in an alarm condition.
On the other hand, the techniques disclosed in the above-cited Japanese Unexamined Patent Publication No. H04-106348, Japanese Examined Patent Publication No. H07-10198, and Japanese Unexamined Patent Publication No. 2010-233304 each concern air-conditioning equipment, but since not only the current on the AC input side of the air-conditioning equipment (i.e., the AC input side of the converter), but the voltage on the DC output side of the converter has also to be monitored, the configuration of the detection mechanism becomes complex, and the installation space and cost increases. Furthermore, the techniques disclosed in the above-cited Japanese Unexamined Patent Publication No. H04-106348, Japanese Examined Patent Publication No. H07-10198, and Japanese Unexamined Patent Publication No. 2010-233304 each involve reducing the motor rotational speed by reducing the AC output frequency, but in the case of a machine tool system, reducing the motor rotational speed results in reducing the machining accuracy. On the other hand, the technique disclosed in Japanese Patent No. 4111138 uses a DC power supply as the input; therefore, the technique cannot be applied to the type of apparatus that converts input AC to DC for output and then converts the DC output back to AC for driving a motor.